In recent years, cloud computing has promoted system aggregation, and thus stable operation of a network has become an important issue. To obtain a stable performance in the network, congestion, that is, traffic collision has to be addressed. Congestion control technology for addressing congestion is preexisting technology, and, for example, there is congestion control technology using an explicit congestion notification (ECN). Japanese Laid-open Patent Publication No. 2006-319746 is one of related art documents.